This proposal focuses on understanding how the major physiological changes occurring during the first postnatal month regulate the sharp changes in expression of key phenotypic cell genes and drive functional maturation of cells. Surprisingly little is known about the later stages of cell maturation. This has assumed practical importance because in vitro human ES-derived cells insulin-producing cells lack the glucose-responsive insulin secretion that is characteristic of a functional cell. Neonatal rodent islets also lack this glucose responsiveness and so serve as a model for the induction of the maturation process. Across the first postnatal month as the glucose-stimulated insulin secretion gradually develops, we found multiple patterns of islet gene expression, most with striking inflection points between P7 and 9 days and again about P15. While we have shown that adenoviral-mediated expression of MafA induced glucose- stimulated insulin secretion (GSIS) almost to adult levels, the developmental enhancement of MafA and other necessary genes must be regulated by physiological stimuli. The dynamic neonatal environment that is very different from that of the adult provides the landscape of these changes: at P1 blood glucose levels are low, as are levels of thyroid hormones (T3 and T4), of corticosterone (the main glucocorticoid of the rat) and of prolactin, and the diet is only milk. Serum T4 levels start changing around P7 and peak at P13 while leptin surges also about P8; there are surges of corticosterone and prolactin around P15 The timing of these hormonal surges suggests that they are the physiological regulators needed for neonatal cells to become functional. To address this question we have three specific aims. Specific aim 1. To determine the mechanisms for thyroid hormone enhancement of cell maturation. We show preliminary data that T3 induces glucose responsive insulin secretion and that is mediated by the transcription factor MafA. We propose experiments to dissect the mechanism of T3's regulation of MafA and other genes induced for functional maturation. Specific aim 2. To determine the importance of the candidate factors T3, leptin, corticosterone and prolactin on cell maturation. The environment of the neonate is very different than that of the adult and dynamic. We hypothesize that various physiological factors regulate the maturation of the cell and are particularly attracted to three hormonal candidates, in addition to thyroid hormone, that may play key roles: plasma leptin concentrations surge at about the P7 inflection point, while corticosterone and prolactin concentrations are gradually increasing until a surge at about P15, the second key inflection point. Specific aim 3. To determine if the physiological factors that are effective in maturing the neonatal rat B cell will similarly direct in vitro functional maturation of the immature human pancreatic progenitors. In this translational aim our goal is to transfer to fetal human pancreatic cells the knowledge of the physiological signals for in vivo maturation gained in the first two aims.